With the development of chemical battery pack techniques, chemical battery packs have rapidly-improved specific energy and gradually-reduced cost per unit. And with features such as rapid response, convenience in setting and the like, the application of battery packs becomes increasingly widespread. As energy supplying devices in digital products, battery packs have features such as portability and high energy density. In connection of new energy resources (such as wind power generation, solar power generation), battery packs are used as energy buffering devices to smooth new energy resources and improve popularity of new energy resources; in automobiles, battery packs are used as devices for recovering braking energy, double as auxiliary power providing devices (generally for HEVs or PHEVs) in special working conditions (such as starting-up, acceleration), or used directly as all-energy resources for automobiles (in the case of PEVs); in micro smart grids, as energy buffering platforms, battery packs are used to balance power generation and power consumption so as to maintain stable and economical operation of a whole grid.
In general, usage temperatures of battery packs are 0° C. to 45° C., within which battery packs exhibit relatively good comprehensive performances such as high power output performance, cycle life, security performance and the like. If an usage environment of a battery pack is at extremely low temperatures or extremely high temperatures, some performances of the battery pack will be degraded remarkably. For example, at minus 20° C., high power output performance of an LCO lithium-ion battery pack (with its positive active material being lithium cobalt oxide and its negative active material being graphite) will be degraded remarkably.
Therefore, in order to deal with working conditions at extremely low environment temperatures, an external environment of the battery pack is usually controlled. For example, in the case of a battery pack used in an EV, in order to enable the battery pack to normally work at low temperatures or high temperatures, a heat management device (such as an air conditioner) is usually provided around the battery pack. However, such a method for controlling environment temperature of the battery pack may generally result in problems of high cost and low efficiency with regard to heat management.
In addition, in some cases (such as ignition during startup of an automobile) where a battery pack works in conditions that high power output is required, the battery pack is required to output a very high power during a very short period of time (about 3 seconds), thus it is not possible for the battery pack to reach a temperature, which enables the battery pack per se to work with high power output, through external heating due to the following reasons: on one hand, it will take a relatively long time and will consume much energy stored in the battery pack; on the other hand, external heating may result in a hidden danger of temperature non-uniformity within the battery pack per se (prolonging heating time can avoid this hidden danger, but excessively long time will defeat the purpose of meeting application requirements, that is, it is impossible for users to accept such a long waiting time). Furthermore, in cases where low temperature high power output is required, low temperature performance of a battery pack can be improved through optimization of battery pack design. However, a change in the chemical system of the battery pack may generally result in a remarkable increase in the battery pack's cost or a remarkable decrease in other performances (such as a decrease in high temperature performance of the battery pack).